Method to linearly measure gas volume flow in ducts and flow sensor

ABSTRACT

A method to linearly measure the flow of a gas in ducts, and a gas flow sensor using such a method are disclosed. In order to obtain a linear ratio between the differential pressure and the volumetric flow of a gas, a rectangular section elbow and a rectangular flexible plate located inside the elbow are used. When the flow goes through the duct linearization is obtained by combining the resistance of the duct and a variable-area obstruction caused by the plate. With linearization, it is possible to expand the measurement range of the gas flow in one duct. The sensor developed is used especially for the measurement of respiratory flow in medical applications.

FIELD OF INVENTION

[0001] The invention comprises the development of a method to obtain a linear ratio between the differential pressure and the volumetric flow of a gas in conducts, and a sensor to measure the gas flow for medical applications using such method.

BACKGROUND

[0002] There are several ways to measure the volumetric flow, for example, using differential pressure, thermal elements, magnetic elements, among others. The sensors used to measure volumetric flow by means of differential pressure include the Verituri Tube, plate-orifice and variable-area obstruction.

[0003] The current variable-obstruction-operating sensors are characterized by using a perpendicularly aligned flexible plate to the direction of the gas flow; when flow is increased, the plate bends generating an increase of the flow path area. This plate has a complex form in order to obtain the linearization between the variation of the differential pressure and the variation of flow.

[0004] Patents U.S. Pat. No. 4,989,456, U.S. Pat. No. 5,038,621, U.S. Pat. No. 5,033,312, EP0373886, EP331772, EP331773, U.S. Pat. No. 4,083,245 and U.S. Pat. No. 4,197,857 reveal variable-area-obstruction flow sensors; due to the complex form of their plates, the variation of the differential pressure with regard to the variation of the volumetric flow is linear.

OBJECTIVES

[0005] To develop a method to linearly measure volume flow in ducts.

[0006] To develop a sensor to measure gas volume flow with the following characteristics:

[0007] Linear ratio between the variation of the differential pressure and the variation of the respiratory volumetric flow.

[0008] Low resistance to flow.

[0009] Measurement of the volumetric flow in both directions.

BRIEF DESCRIPTION OF THE INVENTION

[0010] A method has been developed to obtain a linear ratio between the differential pressure and the volumetric flow of a gas, and a sensor to measure gas flow for medical applications using such method.

[0011] The method allows to obtain a linear ratio between the differential pressure and the gas flow, using a rectangular-section elbow and a rectangular flexible plate, that is, when the flow passes through the duct, linearization is obtained by combining duct resistance and variable-area obstruction caused by the plate.

[0012] The sensor uses this method to linearize the ratio between the differential pressure and the volumetric flow. It has a symmetric structure with regard to its cross section and therefore measures the flow in both directions.

[0013] The design of the sensor includes the elbow described in the method, two elbows joined to the sensor by their ends, a flexible plate and two spigots. These three elbows are consecutive and have a rectangular section, and redirect the flow in three stages; the two elbows jointly have a tubular segment at their ends for connection with other accessories. The rectangular-shaped flexible plate is located within the sensor, and matches the symmetry cross section at rest. The spigots through which the differential pressure signals will be obtained are parallel to each other and are located at the outer elbows, on the same side of the fixed edge of the plate.

[0014] This sensor is especially developed for Pulmonary Ventilators. Given that it measures the flow in both directions, it is used to measure the inspiration and expiration flow.

DRAWING DESCRIPTION

[0015]FIG. 1 shows the diagram of the method.

[0016]FIG. 2 shows the sensor design.

[0017]FIG. 3 shows a cross section of the flow sensor, where the flexible plate may be seen in its rest and working states.

[0018]FIG. 4 shows the flexible plate at rest.

[0019]FIG. 5 shows a cross section of one of the spigots.

[0020]FIG. 6 shows a cross section of one of the tubular segments.

DETAILED DESCRIPTION OF THE INVENTION

[0021] A.—Variable Obstruction Method.

[0022] The various applications require to linearly measure the gas flow to predict its behavior and expand the flow measurement range in ducts.

[0023] The method developed consists in using a rectangular section elbow and a rectangular flexible plate located inside the elbow (FIG. 1), in order to linearize the ratio between the differential pressure and the flow of a gas.

[0024] The variable obstruction method uses the differential pressure to measure the gas volume flow; plate flexibility, duct shape and variation of the flow generate the pressure drop. At low flows, the loss of energy caused by the plate has a greater influence on the differential pressure than the one caused by the duct (slight bending of the plate); whereas, at high flows, the loss of energy generated by the duct shape has a greater influence on the differential pressure (the bending of the plate increases with regard to the bending at low flows).

[0025] B.—Gas Flow Measurement Sensor

[0026] The development of the sensor to measure gas volume flow comprises the use of three elbows, a flexible plate and two spigots, and to obtain such linear ratio the previous method is used. (The explanation will be made by using FIGS. 3 to 6).

[0027] The sensor has a symmetric structure with respect to the cross section A-A (FIG. 3). It is formed by three elbows (5, 6 and 7, FIG. 3). The elbows at the ends (6 and 7) have a 45° angle and a tubular segment at their ends (10, 11), for connection with other accessories, the inflow (10) and outflow (11) segments are aligned to each other, and the intermediate elbow (5) is at a 900 angle. These elbows are used to redirect the flow in three stages.

[0028] This sensor has two spigots (8 and 9) which are parallel to each other, and are located at elbows 6 and 7. They are located on the same side of the rectangular section of the duct or insertion edge (curb) of the plate, and are used to connect a differential pressure transducer therein. The total sensor segment comprised between such spigots has a rectangular section.

[0029] The plate used is made of polymer, is flexible and 0.1 mm thick. It is rectangular-shaped and fixed at the distal curb of the intermediate elbow. FIG. 3 shows the plate at two stages, rest state 2 is shown while there is no flow circulating through the duct. At this stage, the plate matches L the symmetry cross section of the sensor. Stage 3 occurs when gas flow is circulating.

[0030] The rectangular shape and thickness of the plate (depends on the flow range intended to be measured) significantly reduces the vibration of the plate caused by the passing of the flow. The vibration of the flexible plates used in conventional sensors are not desirable because this phenomenon reduces the accuracy and reliability of measurements. In lab conditions, the range of displacement of the vibration of the plate-free edge was below 0.5 mm when the sensor was at the upper limit of the measurement range (maximum flow). Furthermore, the shape and thickness of the plate prevents the breaking of the polymer microscopic chains. This breaking phenomenon causes the plate to lose rigidity with the passing of time, and therefore, measurement repetition.

[0031]FIG. 6 shows that the inflow and outflows segments (10 and 11) are tubular-shaped, and their diameters may be set up according to the desired application to the sensor. The diameters are to be chosen according to standardized values.

[0032] The sensor has a low resistance to gas flow due to the low loss of energy caused by the flexible plate.

[0033] The sensor is used especially to measure the respiratory flow.

[0034] The invention may be developed in other ways without drifting away from its fundamental characteristics.

[0035] The creation described is considered to be only illustrative and not restrictive. Therefore, the scope of the invention is prescribed more based on the following claims than based on the above description. Any change or variation regarding secondary details on what is intended to be conveyed in the claims is comprised within their scope.

SUMMARY OF THE INVENTION

[0036] A method to linearly measure the flow of a gas in ducts, and a gas flow sensor using such method have been developed.

[0037] In order to obtain a linear ratio between the differential pressure and the volumetric flow of a gas, a rectangular section elbow and a rectangular flexible plate located inside the elbow are used. When the flow goes through the duct, linearization is obtained by combining the resistance of the duct and a variable—area obstruction caused by the plate. With linearization, it is possible to expand the measurement range of the gas flow in one duct.

[0038] The sensor developed is used especially for the measurement of respiratory flow in medical applications. This sensor has a symmetric structure with regard to a cross section and is formed by three elbows, two tubular segments, a flexible plate and two spigots. The outer elbows have a 45° angle and the middle elbow has a 90° angle.

[0039] The rectangular flexible plate made of polyethylene is inserted in the distal curb of the 90° elbow, and matches the sensor's symmetrical plane at rest.

[0040] The two tubular segments are colinear and joined to the 45° angle elbow. The two spigots are located in these elbows and are positioned on the same side as the fixed edge of the plate. They are used to place a pressure transducer to take the differential pressure signals.

[0041] The sensor measures the volumetric flow in both directions and the results are approximately the same. This is due to the fact that the plate has freedom of movement in both directions and the sensor is symmetric with regard to its cross section. 

1. A variable obstruction method to obtain a linear ratio between the differential pressure and gas volumetric flow, which consists in using a rectangular section elbow and a flexible plate located on the symmetry cross section of the elbow.
 2. According to claim 1, the method involves a flexible plate at a rest state matching the symmetry cross section of the elbow.
 3. According to claim 1, the method involves a flexible plate fixed on one of its edges to the elbow's distal curb.
 4. According to claim 2, the useful range of the linear ratio is obtained by combining the elbow angle and the rigidity of the plate according to the application required.
 5. A volumetric flow sensor for medical applications with the following characteristics. A symmetric duct with regard to its cross section, with two consecutive elbows and two aligned tubular inflow and outflow segments, and capable of measuring the flow in both directions with similar results. A flexible rectangular plate fixed on one of its edges to the intermediate elbow's distal curb, and matches the symmetry cross section of the sensor at rest.
 6. A flow sensor according to claim 5; the angles of the three consecutive elbows are 45°, 90° and 45°, respectively.
 7. The flow sensor according to claim 5, in which the internal section of the elbows is rectangular.
 8. In the flow sensor according to claim 5, the amplitude of the vibration of the plate-free edge during the passing of the flow is 0.5 mm when the sensor is at the upper limit of the measurement range.
 9. In the flow sensor according to claim 5, the spigots are located on the outer elbows and are parallel to each other.
 10. In the flow sensor according to claim 5, the orifices used by the transducer to measure the differential pressure and the plate fixed edge are on the same side of the rectangular section of the duct. 